The invention relates to the finishing of granular polymer resins. In one aspect, the invention relates to a process for production of finished granular resins of desired bulky density and having additives incorporated therein and improving flowability.
The recent development of certain linear low density polyethylenes (LLDPE) has resulted in a new product which presents certain superior properties over conventional branched low density polyethylene (LDPE). These new resins are manufactured by low pressure processes which produce the resin as granular product large enough and dense enough that conventional pelletizing is unnecessary. This not only saves cost but also avoids any resin degradation which might result from the pellet forming operation during the manufacturing process of the resin.
Since the polyethylene granules do not need to be pelletized, conventional polyethylene compounding and finishing operations are not suited for this product. For example, in pelletizing conventional LDPE, additives (e.g. antioxidants, antiblock and slip, etc.) are conveniently incorporated by melt blending in the mixer and/or extruder used to form the pellets. (The term "finishing" as used herein refers to a process for converting virgin resin to useable form. Additives are incorporated to impart the desired end-use properties to the product and particle shaping and size classification place the product in a form suitable for the fabricators.)
Other polymer finishing techniques, of course, have been suggested by the prior art, but these are not totally satisfactory because of the restrictive requirements for finished polyethylene.
An important requirement of the finishing technique is to disperse the additives throughout the granular product. For best results, the additives should coat or contact substantially all of the granules.
Another important function of the finishing technique is to increase the bulk density of the granular material in order to reduce storage and transportation volumes, as well as maximize extrusion equipment throughput rates. This requires improvement of the overall shape factor by reduction of the average particle size of the granules, while avoiding generation of excessive fines. Finally, the product with additive should be free-flowing discrete particles to permit handling and transporting with conventional equipment.
In providing the desired properties and particle size to the product, the finishing system should be capable of processing the granules at full production rates. For all practical purposes, this means the process should be continuous, as opposed to batch. At commercial rates, batch processes are generally unsatisfactory because parallel alternating systems, and batch cooling are required.
Simply blending or tumbling the ingredients together at ambient conditions is not satisfactory for several reasons. Uniform additive dispersion is not only difficult to obtain, but the "salt-and-pepper" mixture is not as effective as additives actually contacting the individual granules. Moreover, the particulate additives such as antiblock (silica) are extremely fine and would not adhere to the granules except possibly by transient electrostatic attraction. These particles would become segregated during handling and storage which not only would reduce the effectiveness of the additive but in the case of silica antiblock could present health hazards due to the free silica, particularly cristobalite. Another serious deficiency of simple blending operations is that the energy imparted to the granules is insufficient to have a significant effect on particle shape factor.
It has also been proposed to prepare pelletized masterbatch with additives, followed by grinding and blending with the virgin resin. A problem associated with this approach is one of economics; the cost of the preparation of the masterbatch itself is relatively high because of the energy required to melt, pelletize and grind the resin forming the masterbatch. Perhaps a more serious problem inherent in the masterbatch technique is that only the particles of masterbatch contain additive. Tests have shown that best results are obtained with additives in contact with all the granules. The resin granules with which the masterbatch is blended do not contain additives. The finished product, including resin granules and masterbatch particles, may not have uniform properties.
Intensive mixers have been proposed for treating resinous particulate materials. These intensive mixers employ rotating blades to impart high energy to the system, causing the mixing to take place at elevated temperatures. Representative uses of intensive mixers in treating particulate resins are discussed below.
U.S. Pat. No. 3,229,002 discloses the use of an intensive mixer for "polishing" thermoplastic pulverized resin (e.g. polyolefins, nylon, etc.) to improve its flowability and bulk density. The purpose of the treament is to improve flowability and density for molding, coating, and rug backing applications.
U.S. Pat. No. 3,632,369 discloses the use of high intensive mixer for admixing pigment with ground resins. The pigment addition is achieved by operating the intensive mixer at conditions to produce abrasive adherence of the pigment to the polymer particle.
U.S. Pat. No. 3,736,173 discloses the use of a high speed mixer to incorporate a curing agent into polyolefin pelletized granules by penetration and diffusion.
U.S. Pat. No. 3,997,494 discloses the use of high speed intensive mixer for incorporating filler material into polymer pellets, then removing the blended materials from the pellets until the filler material is used up.
U.S. Pat. No. 4,230,615 discloses the use of a high speed high intensity mixer to fully flux thermoplastic resins.
As will be apparent from the following description, the method of the present invention involves principles of operation different than the techniques noted above, and accordingly, represents an improvement thereover for treating granular resins. The invention permits the finishing granules of thermoplastic resins without fluxing or grinding the resin.